1. Field of the Invention
This invention relates generally to vehicle power drive trains and more particularly to a DC motor and hydraulic system which enables an electric motor driven lift truck to have both sufficient torque to climb grades and sufficient speed on both grades and horizontal surfaces to make it practical.
2. Description of the Related Art
Lift trucks, sometimes referred to as forklifts, have long been recognized as having a high utility for transporting merchandise and industrial equipment. They are typically propelled by internal combustion engines or electric motors, most commonly DC electric motors, powered by storage batteries. FIG. 1 illustrates a lift truck.
Lift trucks driven by DC electric motors typically have electrical control systems which vary the voltage applied to the DC motor and in that manner, control the speed at which the lift truck is propelled. A block diagram of a drive system for a typical lift truck driven by a DC motor is illustrated in FIG. 2. Although DC motor driven lift trucks have a transmission for providing a desirable torque/speed ratio between the DC motor and the driven wheels of the lift truck, typically such transmissions do not have a plurality of selectable gear ratios. Instead, speed is ordinarily controlled only by controlling the rotational speed of the DC motor by varying the voltage applied to it.
Lift trucks powered by internal combustion engines utilize a conventional engine throttling system to control engine speed and therefore vehicle speed, but also have mechanical transmissions followed by drive links to the wheels for selecting major step changes in gear ratios between the engine and the wheels. Such a prior art lift truck drive system is illustrated in FIG. 3. There may, of course, be additional components interposed in the drive train of both types of conventional lift truck drive systems as it extends from the power plant to the driven wheels.
Prior art lift trucks also have utilized hydraulic pump and hydraulic motor systems. Commonly, an internal combustion engine drives a hydraulic pump, which applies hydraulic fluid under pressure to one or more hydraulic motors, typically an individual hydraulic motor for each wheel. Such systems are provided with hydraulic controls for varying the flow rate and/or pressure to each hydraulic motor for controlling the speed of the vehicle and, in some instances, for controlling steering.
Electric lift trucks have the desirable characteristic that they do not emit noxious fumes and pollutants. However, they suffer from the disadvantage that they do not have as much power output as the internal combustion engine systems. The consequence is that electric lift trucks cannot be used in a work environment where they must travel up ramps, hills or other grades. Electric lift trucks have been unable to continuously climb a grade, but instead can go only as far as their momentum can carry them. In those lift truck applications which require the ability to travel up a grade, the internal combustion engine systems are necessary, but the pollutant emissions can be a problem. An engineering consultant has observed that it was not possible to build a commercially practical and acceptable electric driven lift truck which had both sufficient torque to accommodate the load demands of travelling up a grade of a magnitude typically encountered by lift trucks and yet also had sufficient grade and horizontal surface speed performance to be acceptable to customers.
It is therefore an object and feature of the invention to provide a DC motor driven lift truck, which is not only economically competitive with other lift trucks, but also capable of operating characteristics which include both a sufficient torque to climb typical industrial grades and sufficient speed to allow practical use.
A further object and feature of the invention is to provide an electric lift truck which does not require the costly DC motor speed controls which control motor voltage, draws less current from the batteries than conventional electric lift trucks so that the batteries last longer before recharging is required, and requires less maintenance costs than conventional DC motor systems.
Yet a further object and feature of the invention is to provide a DC motor driven lift truck, which not only exhibits the above characteristics, but additionally provides a very broad range of torque/speed drive ratios between the electric motor and the driven wheels of the lift truck.
The lift truck of the invention has a power drive train, including the combination of: an electric motor which is preferably operated continuously at full voltage, a displacement hydraulic pump driven by the electric motor and having a pressure output capacity of at least 3,000 psi; a displacement hydraulic motor hydraulically connected to the pump and having a rotational speed capacity of at least 3,000 rpm; and a hydraulic fluid volumetric flow rate control connected in the hydraulic system for controllably varying the torque and speed of the hydraulic motor.